Multi-user multiple-input multiple-output (MIMO) transmission using a grid-of-beams (GoB) approach has been shown to be an attractive scheme for space-division multiple access (SDMA) for emerging wireless systems. See, for example, IST-4-027756 WINNER II Deliverable D4.7.3, “Smart antenna based interference mitigation,” June 2007 (hereinafter “WINNER II”). In a GoB scheme, a grid of beams is created by using a closely spaced array of antennas at the base stations. Independent data streams are transmitted to mobile terminals in geographic locations served by non-overlapping beams. A hallmark of this scheme is that it requires very little channel state information at the transmitter (CSIT), as the system need only select an appropriate one of the grid of overlapping beams to serve a particular mobile terminal.
While the GoB-SDMA approach relies on fixed beams, the steering of antenna beams by means of baseband signal processing is also well known. This approach can provide improved coverage and less interference, at the expense of more complex processing and more complicated channel state feedback mechanisms. The general problem of joint adaptive beamforming from a multi-antenna base station to multiple single-antenna mobile terminals has been solved. See, for example, M. Schubert and H. Boche, “Solution of the multi-user beamforming problem with individual SINR constraints,” IEEE Trans. Vehicular Technology, vol. 53, no. 1, January 2004 (hereinafter “Shubert”). With Shubert's solution, the beamformers and transmission powers are jointly adjusted to fulfill individual signal-to-interference-plus-noise ratio (SINR) requirements at the mobile terminals. Shubert presents an algorithm that maximizes the jointly-achievable SINR margin (over the SINR requirements) under a sum transmit power constraint, i.e., a constraint that limits the total power transmitted from all the antennas of the base station antenna array. Shubert also provides an alternative algorithm that minimizes the sum transmit power while satisfying the set of SINR requirements for the mobile terminals. Each of these algorithms requires the base station to have statistical information characterizing the channel conditions.
A coordinated multi-point (CoMP) transmission system architecture is being considered for IMT-Advanced systems. (The term “distributed antenna system,” or DAS, has also been applied to these concepts under development for IMT-Advanced systems; the term CoMP, as used herein, is not intended to exclude systems or techniques that use the DAS nomenclature.) CoMP differs from a conventional cellular system in that antennas are deployed at several access points dispersed across a CoMP cell. The access points are connected to a central processing unit (CPU) by means of a fast backhaul. Compared to a conventional cellular network, very high spectral efficiencies are possible in a CoMP network. In the downlink, this is because transmissions from multiple access points can be coherently coordinated. In the uplink, mobile terminal transmissions can be received at multiple access points, and the received signals processed jointly for improved reception. Of course, this coordination requires that statistical information characterizing the channel conditions between a given mobile and each of the access points is available.
The fixed and adaptive multi-user beamforming schemes applied to conventional cellular systems are not appropriate for a CoMP network, as these schemes cannot exploit the coordination between access points in a CoMP cell. Furthermore, applying beamforming schemes in a CoMP network can create intra-cell and inter-cell interference that are not easily moderated using conventional interference cancelling schemes.